1. Field of the Invention
The present invention relates to intake air amount measuring systems for internal combustion engines, and more particularly the invention is related to a system for measuring the amount of intake air flow to internal combustion engines of the type incorporating an electronically controlled fuel metering system.
2. Description of the Prior Art
Controlling the air-fuel ratio of mixtures supplied to an internal combustion engine at a predetermined ratio is a very effective means of purifying the exhaust gases discharged from the engine. For this purpose, it is necessary to accurately measure the amount of intake air flow to the engine and supply fuel to the engine in an amount corresponding to the intake air amount. A known measuring system of this type employs the method of indirectly measuring the amount by volume of intake air in accordance with the engine rotational speed and intake vacuum or the engine rotational speed and throttle valve opening angle.
While the measuring system employing the above-mentioned measuring method has some advantages, there is also a disadvantage that since the amount of air drawn from the intake pipe is measured indirectly, the error in measurement tends to become large due to the effects of variations among engines caused during the manufacturing process, deterioration of engine, intake and exhaust valve clearances, deterioration of air cleaner with time, etc. Another disadvantage of this method is that since the amount of intake air is measured in terms of volumetric flow rate, conversion into absolute pressure must be effected thus making the system more complicate and expensive. Another prior art system such as disclosed in U.S. Pat. No. 3,975,951 has been proposed to overcome the foregoing difficulty. More specifically, the system is so constructed that a branch having two passages is disposed in the intake pipe of an engine, and first and second thermally-sensitive resistors are disposed in the passages in such a manner that a bridge circuit is formed by the thermally-sensitive resistors together with two separately provided reference dividing resistors. An electric heater is positioned upstream of the first thermally-sensitive resistor disposed in one of the passages in the branch, so that the heat generated by the electric heater will produce a temperature difference between the passages in the branch. This temperature difference causes a resistance differrence corresponding to the temperature difference between the first and second thermally-sensitive resistors. Thus, by controlling the voltage applied to the electric heater or the amount of heat generated by the electric heater in such a manner that a voltage difference caused by the resistance difference is always maintained constant, that is, the temperature difference between the first and second thermally-sensitive resistors is always maintained constant, it is possible to cause the said voltage to assume a value corresponding to the amount of intake air. While the inventors have made this prior art system in which the amount of intake air flowing in the intake pipe is directly measured in terms of weight flow, this intake air amount measuring system is disadvantageous in that if the electric heater is electrically connected in parallel with the bridge circuit and the generation of heat by the electric heater is controlled to maintain the temperature difference between the two passages in the branch constant, that is, if the same d.c. voltage is applied to the electric heater and to the bridge curcuit whose output is amplified by a known type of d.c. differential amplifier, and the amount of heat generated by the electric heater is controlled through the voltage applied to the electric heater, a drift is caused in the bridge circuit by the fact that current is always flowing in the bridge circuit. Another problem with this prior art system is that since the amplifier is a direct-coupled amplifier, a temperature drift is caused in the amplifier thus making it impossible to accurately measure the amount of intake air. Generally, while, in realizing a low drift circuit, it has been the practice to use expensive low drift elements as a measure to reduce the drift, this inevitably tends to increase the manufacturing cost as compared with a circuit employing general-purpose elements.